Thermal ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an ink-jet pen.
In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble in the chamber, which expels ink through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
Many inks that are described for use in ink-jet printing are usually associated with non-thermal ink-jet printing. An example of such non-thermal ink-jet printing is piezoelectric ink-jet printing, which employs a piezoelectric element to expel droplets of ink onto the print medium. Inks suitably employed in such non-thermal applications often cannot be used in thermal ink-jet printing, due to the effects of heating on the ink composition.
Colored inks made using heavy dye loads, when printed on bond paper, tend to bleed and give reduced waterfastness. Bleed, as used herein, is the invasion of one color. into another color on paper, which is a surface phenomenon. This is in contradistinction to uses of the term in the prior art, which tend to define "bleed" in the context of ink of a single color following the fibers of the paper; this is a sub-surface phenomenon, and is called "feathering" or "threading" herein. Waterfastness, as used herein, refers to resistance to washing out with water after printing on paper.
Surfactants have been used as anti-clogging agents in Japanese Laid-Open Patent Application No. 63-165465 for use in ink-jet recording inks. The surfactants used in that application are limited to those having a surface tension between 20 and 50 dyne/cm. The amount of surfactant ranges from about 0.5 to 25 wt %. Specific examples disclosed include sodium dodecyl benzene sulfonate, sodium laurate, and polyethylene glycol monooleyl ether.
Japanese Laid-Open Patent Application No. 01-203,483 is directed to ink-jet recording compositions. Bleed reduction is mentioned in connection with printing using the inks. The compositions also require pectin (0.01 to 2 wt %), which is probably being used as a thickener. However, pectin is not useful in inks used in thermal ink-jet printers, due to its thermal instability (it gels at higher temperatures).
Japanese Patent JO 1215-875-A is directed to inks suitable for ink-jet printing, evidencing good recording with fast drying without bleeding. The compositions all require triglycerides. Such compounds, however, are not stable to extended shelf life necessary for commercial ink-jet printing.
Japanese Patent JO 1230-685-A is directed to inks suitable for ink-jet printing, evidencing quick absorption on the surface of conventional office paper without smear or blotting. The compositions comprise colorants and liquid solvents and/or dispersants and are characterized by the presence of a copolymer of ethylene oxide and propylene oxide of the formula HO(C.sub.2 H.sub.4 O).sub.a -C.sub.3 H.sub.6 O(C.sub.2 H.sub.4 O).sub.b H, where a+b is up to 50 and b is optionally 0. These copolymers are referred to as "PLURONICS". For the most part, they have not been found to stop bleed or to give good print quality.
High molecular weight (&gt;10,000) colloids, such as sodium alginate, have been used to control bleed, as disclosed and claimed in copending application Ser. No. 07/737,101, now U.S. Pat. No. 5,133,803, filed Jul. 29, 1991, and assigned to the same assignee as the present application. The high molecular weight colloids, used in conjunction with water-soluble dyes, require the presence of at least one zwitterionic surfactant and/or non-ionic amphiphile.
Good line definition has been afforded in using the following formula for thermal ink-jet inks (cyan, magenta, yellow):
8 wt % 1,5-pentanediol; PA1 2 wt % SURFYNOL 465 (a trademark of Air Products & Chemicals, Inc.); PA1 0.25 wt % sodium alginate (high molecular weight); PA1 1.0 wt % N,N-dimethyl-N-(Z-9-octadecenyl)-N-amine oxide (OOAO); PA1 Dye, such as 0.75 wt % sodium-Acid Blue 9, 1.2 wt % sodium-Acid Red 52, or a mixture of 0.25 wt % Direct Yellow 86+0.25 wt % Acid Yellow 23; and PA1 the balance water.
These inks evidence alleviation of both threading and bleed between colors printed one adjacent the other. However, they are not waterfast.
To achieve waterfastness, for example, water-insoluble dyes have been used along with cyclodextrin; application Ser. No. 07/702,437, now U.S. Pat. No. 5,108,505, filed May 16, 1991, and assigned to the same assignee as the present application, discloses inks containing water-insoluble dyes and cyclodextrin. It is currently believed that the highest degree of waterfastness will come by the use of water-insoluble dyes.
Microemulsions, which may be defined as thermodynamically stable isotropic "solutions" of water, oil, surfactant, and co-surfactant, have been used to solubilize water-insoluble dyes for ink-jet printing in the past. The function of water is to provide a continuous phase for the microemulsion droplets and it facilitates the formation of microemulsion droplets by entropic means. The oil is a water-insoluble substance which resides primarily in the microemulsion droplets--the discontinuous phase. The surfactant is an amphipathic, surface active, self-aggregating species which is primarily responsible for the formation of microemulsion droplets. The co-surfactant is an amphipathic species which significantly concentrates in the microemulsion droplets and it affords stability to the droplets.
The success of microemulsion-based inks has been limited due to the extent of threading or feathering in the resulting print sample. Apparently, the combination of high organic solvent and surfactant concentrations used in these inks causes extensive wetting of the paper fibers and fillers, resulting in feathering; poor edge acuity is the end result.
Attempts have been made to overcome such print quality deficiencies by the use of microemulsion-based inks that are solids at ambient temperatures, but are liquids at elevated (e.g., 70.degree. C.) temperatures; see, e.g., U.S. Pat. No. 5,047,084. These inks, however, place additional demands on the printhead and the printer, such as pre-heaters to keep the ink in liquid form prior to firing, and rollers to flatten the solid ink droplets (lenslets) that are formed on the print medium, thus making the product more complex and costly.
Hence, the use of completely water-insoluble dyes in liquid inks, which are completely waterfast when printed on print media, makes their use an attractive approach. Thus, a need remains for colored inks in which threading and bleed are alleviated, yet which are also waterfast and afford high print quality printing.